Photochromic substances are such materials whose molecules reversibly isomerized when they are irradiated with light.
One of the photochromic substances conventionally studied by many people is diarylethene. FIG. 15 shows the structural formula of a diarylethene molecule. Diarylethene turns to a colored state when it is irradiated with ultraviolet light and to a faded state (i.e. the original state) when it is irradiated with visible light. In addition to this photochromic property, diarylethene has the following characteristics:                It has a high quantum yield. That is, it is quick to change its color and highly colorable.        It is very reactive when it is in a polymer. Therefore, it can be easily fused with various materials.        It is durable. The coloring and fading properties scarcely deteriorate even after the coloring and fading reactions are repeated ten thousand times.        It is thermally stable. Its half-life from the colored state to the faded state is approximately 2000 years.        
Due to these characteristics, diarylethene is suitable for application to optical recording materials and there are various research and development activities underway, aiming at its practical use. For example, Japanese Unexamined Patent Application Publication No. 2005-325087 discloses an optical functional device employing multiphoton absorption using diarylethene compounds.
As stated previously, diarylethene is thermally very stable. In other words, it is very slow to be thermally faded. This means it will virtually never undergo natural fading.
If diarylethene or a similar photochromic substance having a high quantum yield, high durability and high thermal stability has a higher thermo-fading speed, the substance will have greater possibilities as dimmer materials for sunglasses and other products.
Efforts have been made for researching a photochromic substance having a higher thermo-fading speed. One example is silver salt, which is used as a material for sunglasses that become colored when they are irradiated with visible light. However, silver salt can be used only for glass. In recent years, with the increasing use of plastics as industrial materials, organic photochromic substances have been demanded. Candidates of such substances include photochromic molecules of a spironaphtooxazine group. However, this substance is low in colorability and weak on durability.
On the other hand, it has been also attempted to provide a derivative of diarylethene with thermo-fadability. It is known that introduction of a branched alkoxy group as a substituent shortens the thermo-fading time of the derivative to approximately 20 seconds. From practical viewpoints, however, the period of 20 seconds is still too long as the thermo-fading time. For example, this photochromic compound is unsuitable for sunglasses because wearing such sunglasses will be dangerous for car drivers, especially when the car is on a road with many tunnels. Moreover, fading this compound requires a temperature of 100 degrees Celsius or even higher, which is impractical. Any photochromic substance to be used in sunglasses or visors under normal conditions must have a thermo-fading time shorter than 10 seconds at room temperature.